MONSOON (Arabic Mausim, season), the name given to seasonal winds due to differences of pressure between areas of land and sea, which are primarily caused by seasonal differences of temperature. Monsoons may be regarded as the seasonal analogue of the diurnal land and sea breezes. The term is, however, also applied to seasonal winds which change in direction on account of the migration of wind-belts in the planetary circulation. During the season of rising temperature the surface of the land warms more quickly, and becomes hotter than that of the sea, and during the season of falling temperature the reverse is the case. Barometric pressure tends to be higher over the colder region than over the warmer, and there is accordingly a tendency for air to flow, in the lower levels of the atmosphere, from the former to the latter. Thus there is in general a movement from land to sea during the cold season, and from sea to land during the warm season.

Within a belt extending from 10 to 15 degrees on each side of the equator, seasonal changes of temperature are insufficient in range to permit of this occurrence of temperature differences adequate to the development of true monsoons. In the higher latitudes of the west wind-belt, and in the polar zones, the generally low temperature does not favour the occurrence of wide differences between land and sea. Thus the conditions required for the occurrence of monsoonal winds are best satisfied in intermediate latitudes in the neighbourhood of the tropics. But, as in the case of land and sea breezes, the strength and extension of the monsoon produced by the action described depends to a large extent on the configuration of the land surface. When the land area consists of a low plain, or of a plateau having a steep coastal strip of small width, the circulation upon it tends to be local, and to approximate to the typical "continental" climate of the temperate zones. Where, on the other hand, the land slopes up gradually to a central massif or ridge the effect of the differences of temperature is, as it were, cumulative, and the monsoons may extend over large areas, affecting regions distant from those in which the causes producing them are directly operative, and the monsoon winds may develop great strength. Ferrel (Popular Treatiseon the Winds) has compared the conditions in the two cases to those of a stove with a long horizontal flue and with a vertical or inclined flue of the same length.

It is of course to be noted that the hot season monsoon is in general of greater strength than that of the cold season, because being usually a sea wind the air is fully charged with moisture, condensation takes place as ascensional movement sets in on reaching the land, and the latent heat set free strengthens the upward current.

The position, outline and relief of the continent of Asia favour the development of monsoons to a much greater extent than any other part of the world; so much so that the climate of the whole of the southern and eastern parts is entirely controlled by these winds, forming what is typically known as "the monsoon region," a region having distinctly characteristic products. Monsoons form an important element in the climate of Australia, western and southern Africa, and the southern part of the United States of America, but with a few exceptions the monsoons of those regions are local in character, modifying the prevailing winds of the planetary circulation (usually the trade winds) for a shorter or longer period every year.